8.2 Hanging Colons

Using a mechanism similar to brace hanging (see Hanging Braces),
colons can also be made to hang using the style variable
c-hanging-colons-alist: when a colon is typed, CC Mode
determines its syntactic context, looks this up in the alist
c-changing-colons-alist and inserts up to two newlines
accordingly. Here, however, If CC Mode fails to find an entry for a
syntactic symbol in the alist, no newlines are inserted around the
newly typed colon.

User Option: c-hanging-colons-alist

The Key: the syntactic symbol

The syntactic symbols appropriate as keys in this association list
are: case-label, label, access-label,
member-init-intro, and inher-intro. See Syntactic Symbols. Elements with any other value as a key get ignored.

The associated value: the “ACTION” list

The action here is simply a list containing a combination of the
symbols before and after. Unlike in
c-hanging-braces-alist, functions as actions are not
supported; there doesn’t seem to be any need for them.

In C++, double-colons are used as a scope operator but because these
colons always appear right next to each other, newlines before and after
them are controlled by a different mechanism, called clean-ups in
CC Mode. See Clean-ups, for details.